1. Field of the Invention
The present invention relates to pharmaceutical compositions and methods for preventing skin tumor formation and causing regression of existing skin tumors. More specifically, the present invention relates to methods for treating precancerous lesions and skin cancer which include administering to a patient an effective amount of indole carbazole compounds as well as to pharmaceutical compositions containing indole carbazole compositions and a pharmaceutically acceptable excipient.
2. Description of Related Art
Previous approaches to prevent or treat epithelial cancers have relied on two strategies. In one, toxic drugs are used which generally interfere with DNA synthesis directly and kill both normal and tumorous cells. The mode of killing is by a direct cytotoxic mechanism. A second technology which has been in use previously is the prevention of tumors by treatment with retinoids, analogs of Vitamin-A. The mechanism of action of retinoids is not understood, but the influence appears to be suppressive rather than curative. There is no induction of the normal terminal differentiation pathways and upon withdrawal of the retinoids, tumors return. Therefore, it is desired to discover methods for treating epithelial cancers as well as precancerous lesions which utilizes a normal physiologic pathway so as to target tumorous cells. It is further desired to discover treatments which cause terminal differentiation of tumor cells so as to have a curative rather than merely suppressive effect.
The mouse skin carcinogenesis model has provided insights into both early events in the development of epithelial neoplasia as well as later changes associated with neoplastic progression. The ability to cultivate keratinocytes in vitro has further facilitated analysis of both genetic and epigenetic events that are important in cutaneous neoplasia. For example, the tumor promoter TPA3 is a potent inducer of maturation in cultured primary mouse keratinocytes but not neoplastic cells. This differential response in vitro may provide insight into the mechanism by which phorbol esters promote tumor formation in vivo. Repeated application of TPA to initiated mouse skin is associated with accelerated maturation and desquamation of normal keratinocytes which permits the clonal expansion of differentiation-resistant initiated cells into a papilloma. An important corollary to this observation is that restoring the ability of neoplastic keratinocytes to differentiate would render them non-tumorigenic.
PKC is composed of a family of phospholipid-dependent kinases which phosphorylate proteins at serine and threonine residues. Since phorbol esters bind and activate PKC, this enzyme has been implicated in TPA-mediated responses such as the induction of differentiation markers in primary keratinocytes. Several classes of inhibitors have been used in an attempt to better understand the functions of PKC; unfortunately, all of these also inhibit other protein kinases which complicates the interpretation of results using intact cells. One of the most potent inhibitors presently available is staurosporine (Tamaoki, T. et al, “Staurosporine, a potent inhibitor of phospholipid/Ca++dependent protein kinase,” Biochem. Biophys. Res. Commun., 135: 397–402, 1986), which inhibits PKC at nanomolar doses in vitro by interacting with its catalytic domain (Nakadate, T. et al, “Comparison of protein kinase C functional assays to clarify mechanisms of inhibitor action,” Biochem. Pharmacol., 37: 1541–1545, 1988; and Gross, J. L. et al, “Characterization of specific [3H]dimethylstaurosporine binding to protein kinase C,” Biochem. Pharmacol., 40: 343–350, 1990). Whereas staurosporine blocks the effects of TPA in several non-epithelial cell types (Sako, T. et al, “Contrasting actions of staurosporine, a protein kinase C inhibitor, on human neutrophils and primary mouse epidermal cells,” Cancer Res., 48: 4646–4650, 1988; Ederveen, A. G. H. et al, “Dissimilar effects of the protein kinase C inhibitors, staurosporine and H-7, on cholecystokinin-induced enzyme secretion from rabbit pancreatic acini,” Eur. J. Biochem., 193: 291–295, 1990: Vegesna, R. V. et al, “Staurosporine inhibits protein kinase C and prevents phorbol ester-mediated leukotriene D4 receptor desensitization in RBL-1 cells,” Mol. Pharmacol., 33: 537–542, 1988; and Watson, S. P. et al, “The action of the protein kinase C inhibitor, staurosporine, on human platelets. Evidence against a regulatory role for protein kinase C in the formation of inositol triphosphate by thrombin,” Biochem. J., 249: 345–50, 1988), it fails to inhibit TPA-mediated maturation in primary mouse keratinocytes and induces certain responses characteristic of TPA exposure (Sako T. et al, “Contrasting actions of staurosporine, a protein kinase C inhibitor, on human neutrophils and primary mouse epidermal cells,” Cancer Res., 48: 4646–4650, 1988).
Previously, it has been reported that systemic administration of staurosporine to mice which were injected with human bladder carcinoma cells, could retard tumor growth by approximately 60% at 1/10 or 1/20 the maximum tolerated dose. Meyer et al, “A derivative of staurosporine (CGP 41 251) shows selectivity for protein kinase C inhibition and in vitro anti-proliferative as well as in vivo anti-tumor activity,” Int. J. Cancer, 43, (1989), pp. 851–856). Using a cell culture assay, Schwartz et al (“Inhibition of Invasion of Invasive Human Bladder Carcinoma Cells . . . ,” J. of the Nat. Cancer. Inst., Vol. 82, No. 22, Nov. 21, 1990, pp. 1753–1756) suggested that staurosporine might inhibit tumor cell invasion of bladder cancer cells but this was not confirmed in vivo. Obrian et al (“Staurosporine: A Prototype of a Novel Class of Inhibitors of Tumor Cell Invasion?,” Editorial, J. of the Nat. Cancer Inst., Vol. 82, No. 22, Nov. 21, 1990, pp. 1734–1735) state that staurosporine may have antitumor activity because of its ability to inhibit protein kinases. Meyer et al, Schwartz et al and Obrian et al all fail to disclose the use of the staurosporine in treating cancerous or precancerous conditions of the skin.
In view of the above, it is desired to obtain a method for treating precancerous and cancerous conditions of the skin which is both effective and curative.